The present invention relates generally to leveling systems and more particularly to automatic leveling systems for off-road equipment, such as mobile vehicles for felling and handling timber. Off-road equipment for use in forestry operations and other off-road construction or excavating equipment generally comprises a tracked or wheeled carrier base having a cab or swing-house assembly rotatably mounted thereto. A boom assembly, including any necessary apparatus for felling and handling trees or for other operations, is operably mounted to the cab assembly. The cab assembly further includes an engine, hydraulic pump system and controls for operating the equipment.
Such off-road vehicles are often required to operate on uneven terrain. Accordingly, the carrier base of the vehicle may not be level when the vehicle is positioned at the desired location for performing work. Inclination of the base causes the center of gravity of the vehicle to shift away from the point of maximum load stability. Thus, the vehicle becomes unbalanced and may overturn unless adjustments are made to reduce the deviation of the center of gravity from the point of maximum load stability.
It will be appreciated by those skilled in the art that, in order to avoid the safety concerns associated with operating the vehicle on uneven terrain and the general discomfort to the operator, it is desirable that the cab assembly be substantially horizontal during operation of the vehicle. To this end, there have been several attempts to design leveling mechanisms and systems for adjusting the orientation of the cab assembly with respect to the base of the vehicle.
One such attempt was disclosed in U.S. Pat. No. 5,337,847, issued to Woods, et al. on Aug. 16, 1994, which is incorporated by reference as if fully set forth herein. The Woods, et al. invention is directed to a four-way leveling mechanism for an off-road vehicle for maintaining a superstructure of an off-road vehicle generally horizontal while the vehicle is working on uneven terrain. The tilt mechanism incorporates a spherical bearing mounted on a shaft perpendicular to the longitudinal pivot axis of the tilt mechanism to eliminate end-play in the joint which forms the longitudinal pivot axis. The longitudinal and transverse pivot axes lie in the same plane so that many of the forces which are transferred from the superstructure of the vehicle through the tilt mechanism to the chassis of the vehicle cannot cause a torque on the components of the tilt mechanism. The tilt mechanism provides for independent adjustment of the angle of tilt relative to each of the two pivot axes.
U.S. Pat. No. 4,991,673, issued to Ericsson on Feb. 12, 1991, which is incorporated by reference as if fully set forth herein, is directed to a cross-country vehicle or machine. The vehicle includes a mobile chassis and a superstructure carrying a driver's cab wherein the superstructure is mounted on the chassis for tilting movement about a horizontal axis and a transverse axis. A hydraulic cylinder on each side of the longitudinal axis acts between the chassis and the superstructure to tilt the superstructure about either or both axes in order to position the superstructure horizontally as the vehicle moves over rough ground.
U.S. Pat. No. 4,679,803, issued to Biller, et al. on Jul. 14, 1987, which is incorporated by reference as if fully set forth herein, is directed to an apparatus for maintain stability of mobile land vehicles on sloping terrain. The vehicle includes apparatus for providing multi-directional relative movement between a conventional tread assembly of the caterpillar type and a swing-house assembly which includes a boom sub-assembly supporting tree-felling and handling equipment, so that the swing house assembly is maintained in a substantially level relationship with the horizontal, regardless of the inclination of the ground on which the vehicle rides, so that the center of gravity of the vehicle is optimized by weight transfer corresponding to the type of ground inclination encountered. The leveling system of Biller, et al. is manually operated.
U.S. Pat. No. 4,650,017, issued to Pelletier, et al. on Mar. 17, 1987, which is incorporated by reference as if fully set forth herein, is directed to a crawler-mounted machine for travel over natural terrain. The vehicle includes a mobile carrier of any conventional structure and a work base mounted over the carrier. A tilt mechanism joins the work base to the carrier for relative tilting movement of the work base with respect to the carrier about a pair of orthogonal axes to hold the work base level.
U.S. Pat. No. 3,835,949, issued to Whelan on Sep. 17, 1974, and incorporated by reference as if fully set forth herein, is directed to a hydraulic leveling-control system. The system automatically maintains a height of a vehicle for all load conditions up to maximum load. The system basically involves a control valve which responds to height signals from hydraulically-operated levelers to direct fluid to the levelers when leveler chain charge pressure is needed to increase vehicle height, or to a circuit bypassing the levelers when the vehicle is at or above level height.
U.S. Pat. No. 5,159,989, issued to Claxton on Nov. 3, 1992, and incorporated by references fully set forth herein, is directed to an automatic hydraulic leveling system. The leveling system includes a low-pressure, high-flow source of hydraulic fluid which is connected for one-way flow into the head end of all four of the cylinders to lower outrigger pads of the vehicle to the ground and then maintain them in contact with the ground. A level sensor senses the relative levelness of the right-front and left-rear corners of the vehicle and high-pressure, low-flow hydraulic fluid is connected to the head end of the cylinder at the lower of those two corners to raise that corner of the vehicle relative to the ground. Likewise, another level sensor senses the relative levelness of the left-front and right-rear vehicle corners and the high-pressure, low-flow hydraulic fluid is connected to the head end of the cylinder at the lower end of those two corners to raise that corner of the vehicle relative to the ground.
U.S. Pat. No. 4,746,133, issued to Hanser, et al. on May 24, 1988, and incorporated by reference as if fully set forth herein, is directed to an automatic leveling system for use in a vehicle having a front, a back, and sides to level the vehicle relative to gravity. The system includes a plurality of devices, each disposed at an individual position relative to the vehicle and in a displaced relationship to the other devices for adjusting the vertical position of the vehicle relative to gravity in accordance with the operation of pairs of adjacent devices. A sensor is disposed relative to the vehicle for sensing the tilting of the vehicle from the level disposition relative to gravity. A system further includes an operating structure responsive to the sensor for operating pairs of adjacent devices in a particular order to obtain an adjustment in the vertical position of the vehicle from side to side and front to back to obtain a resultant leveling of the vehicle relative to gravity.
U.S. Pat. No. 4,288,196, issued to Sutton, II on Sep. 8, 1981, and incorporated by reference as if fully set forth herein, is directed to a computer-controlled backhoe have dual-access gyro which continuously senses the position of the frame and senses corresponding signals to the computer which adjusts the frame position through hydraulic actuators.
One drawback associated with many of these devices is that they include complex control circuitry and are, thus, expensive to incorporate into off-road equipment. Furthermore, rather than adjusting automatically to changes in the orientation of the vehicle, many of these devices require manual adjustment by an equipment operator in order to achieve proper leveling. What is needed, then, is an automatic leveling system for off-road equipment that incorporates a simplified sensing and control system to automatically level the cab assembly of the vehicle in response to changes in orientation. This device is presently lacking in the prior art.